In neural development and regeneration, a stage (1) in which a nerve cell is developed or regenerated and a stage (2) in which an axon of the developed or regenerated nerve cell is correctly outgrown to correctly form a synapse are required. A highly motile flabellate structure called a growth cone is present at an axon tip of the developed and regenerated nerve cell. The growth cone has a central domain, and a peripheral domain including lamellipodia and filopodia. The growth cone repeats extension and retraction of its filopodia to appropriately guide an axon for synapse formation.
Growth associated protein 43 (neuromodulin) (hereinafter, referred to as “GAP43”), which is abundantly present in a growth cone, plays an important role in filopodia extension and neurite branching. GAP43 is phosphorylated by protein kinase C, and dephosphorylated by calcineurin. Non-phosphorylated GAP43, which functions as an actin capping protein, shows decreased filopodia extension. Further, phosphorylation of a serine residue at position 41 (S41) in mouse GAP43 is thought to be important to stabilize actin which is involved in filopodia extension (for example, see Denny, J B., Curr Neuropharmacol., vol. 4 (12): pp. 293-304 (2006)).
Conventionally, in order to evaluate the stage (2), an anti-GAP43 antibody that recognizes phosphorylated S41 is used.